Tracking email opens is surely helpful to their businesses, but is it really fundamental?

Depends on how you look at it. To me their business models are about understanding their users and exploiting their behaviors for more revenue. Understanding their behaviors in regards to email usage is one of the ways they work. To me "understanding user behaviors' is definitely fundamental

Even if Google did zero tracking of users, they'd still be able to display adverts relevant to the search term used when displaying search results. Tracking you across sites just increases their profit, it's not necessary for their business to exist and be profitable. And even if you argue that it is necessary for them to remain profitable, then they don't deserve to remain profitable. Not at this expense.

It is one of the classic things to track in A/B tests. And A/B testing is a pretty fundamental part of their business.

Flawed logic; A/B tests are hypothesis testing, the specific hypotheses can still violate one's privacy. If they could use a bit of retrofitted code to surreptitiously capture your text messages and geolocation, it would certainly help their business, but would it be ethical?